Your skin is teeming with bacteria. There are billions of them, living on the dry parched landscapes of your forearms, and the wet, humid forests of your nose. On your feet alone, every square centimetre has around half a million bacteria. These microbes are more than just passengers, hitching a ride on your bodies. They also affect how you smell.

Skin bacteria are our own natural perfumers. They convert chemicals on our skin into those that can easily rise into the air, and different species produce different scents. Without these microbes, we wouldn’t be able to smell each other’s sweat at all. But we’re not the only ones who can sniff these bacterial chemicals. Mosquitoes can too. Niels Verhulst from Wageningen University and Research Centre has just found that the bacteria on our skin can affect our odds of being bitten by a malarial mosquito.

The mosquito that carries malaria – Anopheles gambiae – tracks its victims with an acute sense of smell. It can track the plumes of carbon dioxide that we exhale, and it’s also attracted to the mix of smelly chemicals given off by our skin. We know that some people smell much more attractive to mosquitoes than others. Our diet matters – for example, drinking beer can give our body odour an irresistible allure.

From past experiments, we know that our skin bacteria are also important. Human sweat becomes more enticing to A.gambiae after it is incubated with skin bacteria for a few days. Even on their own, the bacteria can produce airborne chemicals that attract mosquitoes.

These studies were done using bacteria cultured in a laboratory; Verhulst wanted to analyse the microbes on real skin. He rubbed glass beads against the feet of 48 volunteers to sample their scents and offered the beads to captive mosquitoes. Nine of the volunteers proved to be mosquito magnets, while seven others were almost invisible to the blood-suckers.

These two groups had important differences in their skin bacteria. The attractive ones had more bacteria on their feet than the unappealing ones, but they also had a narrower diversity of microbes; their communities were larger, but more gentrified.

Verhulst even managed to identify specific bacteria that drew in or put off the mosquitoes. People with lots of Staphylococcus or Variovorax were more attractive, while those rich in Pseudomonas, Leptotrichia, Delftia and Actinobacteria were not.

It’s not hard to see how these studies could eventually help us to develop new chemicals that attract mosquitoes into traps, or repulse them from humans. But there’s a lot of work to do first. For Verhulst and his team, the most obvious next move is to work out which chemicals the different skin bacteria are giving off.

For example, what is it about the presence of Pseudomonas that shields us from the attention of mosquitoes? Do they produce chemicals that actively repel the insects? Do they break down the attractive compounds of other bacteria or mask their effects? Do they crowd out bacteria like Staphylococcus, which produce more appetising scents? And why do people with more diverse skin bacteria smell less inviting? Perhaps among a larger multitude of species, it’s more likely that one of them secretes chemicals that drive mosquitoes away, or interferes with their sense of smell.

Finally, are people with certain skin bacteria actually more likely to contract malaria? And would it ever be possible to reduce the risk of malaria, or other infectious diseases, by actively changing the communities on our skin to alter the way we smell?

Verhulst’s study demonstrates, yet again, that we are more than just a collection of human cells. We’re also the sum of the bacteria and other microbes that live within us. These legions affect the way we smell, and thus our risk of disease. They, in turn, are affected by us: immune system genes can influence our body odour, perhaps by dictating which bacteria can set up shop on our skin. It is a convenient fiction to talk of ourselves as single creatures; we are really more like a super-organism consisting of hundreds of species, only one of which is human.

Comments (6)

Mark O. Martin

This is a very cool story, outlining the cloud of metabolites with which we are surrounded, and the role of the microbes associated with us on its composition. Sometimes the metabolites can act as “signals” in interesting ways. This has been seen in insect-insect interactions, for example: specific microbes living within aphids generate a chemical “cue” (a kairomone) that attracts predators:

Nifty. I can’t wait until a subsequent paper breaks down the results into the mosquito attractiveness of various Staphylococcus species. S. epidermidis uses its quorum sensing system to block S. aureus from the skin, so I wonder if there’s a differential in mosquito attraction between the two species of Staph.

We’ve known for years that the way we smell determines how attractive we are to mosquitos. Years ago, on planning a canoe trip down the Colorado River, my husband began taking extra doses of B-complex vitamins. While his clothing came home smelling like a pharmacist’s, he wasn’t bitten at all, while his companions were covered in bites. And here in the States, at least, a common skin lotion – Avon Skin-So-Soft – is touted as a mosquito repellant as effective as the more noxious DEET concoctions sold over the counter.
That this phenomenon occurs with naturally-occuring skin microbes is very interesting. Will be looking for a “natural” mosquito repelling “germ lotion” to hit the market as soon as they can figure the details out. Only downside I can see is that it might not be suitable for those who are immunocompromised – as with AIDS. Slathering germs all over one’s body might be just asking for an infection. And then there is the question of whether the smell is repellant to humans around you, too. My husband’s sweat was nasty for several weeks after he got home from the river …

It’d be cool to make a lotion to transform the resident bacteria on your skin to the non smelly kind, thereby making you invisible to mosquitoes. I’d imagine it’d be easy once you know which genes code for the smelly proteins.